In an evaluation circuit, the inductor coils are often combined with a capacitor to create an oscillator part where the inductance and the capacitance determine a resonant frequency f. In the case of a position change of the damping part, the coverage of the flat inductor coils changes as well. Due to the eddy current effect, the inductance of the coil changes and, therefore, the resonant frequency f changes. In the end, the evaluation circuit possesses a characteristic curve f(s), in this example the oscillator frequency f is a function of the position s.
It is known in the art to design the flat coils as rectangular coils, hereby obtaining a relatively wide and linear measuring range M. Also known in this context, from an older publication EP 06015898 published as EP 1 887 322 A1 and the appreciated state of the art, is that a trapezoid extension of the rectangular coil end increases the linear and useable measuring range M so that it matches the coil length L. These conditions are the basis of the invention, meaning that the invention is based on this inductive sensor technique, differing from resistive, capacitive, or magneto-resistive and other measuring principles.
However, these rectangular coils have almost exclusively been used in a way that they were configured, one after another, in one single measuring direction. Two triangular interleaved and overlapping flat coils, which are also known in the art, cannot be compared with this invention because they do not possess linear characteristics.
Deviating hereof, as known from FIG. 7 of WO 97/39312 A1 and FIGS. 2 and 13 from US 2002/0186007 A1, coil pairs are known in the art which overlap each other within the measuring range. However, these configurations measure just the inductance, instead of the oscillator frequency, or (if the oscillator frequency is measured as in WO 97/39312 A1), no attention is paid to a linear characteristic.
It is also known that, during the measuring sequence, the generically decisive coil configuration can be exposed to interferences. Changes in the distance of the measuring level of the flat coils and the damping part cause sensitive reactions of the linear characteristics. Also a tilting of the measuring configuration, meaning that different distances between one coil and another coil, can cause measurement errors. Finally, the measuring result is dependent on the temperature. Therefore, the task of this invention is to create a coil configuration, not receptive to interferences.